This invention relates to seismic data acquisition and to methods of processing seismic data. It relates to a process for filtering coherent noise and interference from seismic data by an adaptive beamforming method. In another aspect, it relates to adaptively filtering coherent noise and interference from seismic data while preserving seismic signals with arbitrary spectral content in the frequency-wavenumber domain. In yet another aspect, it relates to adaptively filtering coherent noise and interference that is temporally and spatially nonstationary. In a further aspect, it relates to adaptively filtering coherent noise and interference that has been recorded by a sensor array in the presence of perturbations.
In seismic surveys, a seismic source induces seismic waves at or near the surface of the earth. Explosives, vibrating devices and airguns are examples of seismic sources. These waves propagate through the earth and are reflected, refracted, and diffracted by formations within the earth, and can be detected by a plurality of sensors (typically geophones or hydrophones) at the earth""s surface. Each such receiver monitors the seismic wavefield, which is then recorded. The data received by a receiver and then recorded are collectively called a trace. The collection of traces are stored for further processing to gain information about the earth""s subsurface. Such information is commonly interpreted to detect the possible presence of hydrocarbons, or to monitor changes in hydrocarbon bearing rocks.
Seismic data in general contains coherent noise signals, along with seismic reflection signals. These noise signals, hereafter referred to as the noise, interfere with the interpretation of the seismic signals, and degrade the quality of the subsurface images that can be obtained by further processing. It is therefore desirable to suppress the noise that is present in the recorded data before processing it for imaging.
In land seismics, source-generated noise like ground-roll and air-waves are the dominant noise types, and can lead to severe degradation in data quality. In marine seismics, energy propagating as waves trapped in the water column and near-surface layers is a significant source, as well as swell noise and bulge-wave noise which result from waves propagating along the streamers of receiver devices. Other sources of coherent noise in marine seismics include passing vessels, other vessels acquiring seismic data in the vicinity, or any drilling activity close to the survey area.
An important feature of the so-called coherent noise present in seismic data is the distance over which the noise appears coherent. In many circumstances, the noise is coherent over only a few meters. In other cases, although the noise is mostly coherent, there exists spatially impulse noise. in such cases, filtering methods which have large spatial extent, like the known frequency-wavenumber filtering generate undesirable artifacts, which are mistakenly identified as seismic events after further processing and imaging.
Another feature of the noise present in seismic data is that it is often non-stationary as a function of time; i.e. its characteristics change as a function of time.
During recent years there have been suggested a variety of methods employing the central concept of applying adaptive signal processing ideas to the problem of suppressing coherent noise in seismic data. Booker and Ong, in: xe2x80x9cMultiple-constraint adaptive filtering,xe2x80x9d Geophysics, Vol.36, pp. 498-509, 1971, have derived an algorithm for multi-channel time-series data processing, which maintains specified initial multiple filter constraints for known signal or noise sources while simultaneously adapting the filter to minimize the effect of the unknown source field. The constraints are of the multiple look-direction constraints type, where look-directions must be precisely specified.
In the International Patent Application WO97/25632, Ozbek has described a class of adaptive signal processing techniques for attenuation of dispersive, nonstationary and aliased coherent noise in seismic data, in the presence of phase and amplitude perturbations. The methods developed can be classified as multi-channel adaptive interference cancellers. Since a signal-free noise reference is not readily available in seismic data acquisition, various preprocessing techniques are introduced to generate the coherent noise reference channels. In the single-component version of the method, moveout (apparent velocity) and spatio-temporal coherence are used as the criteria for differentiating between the signal and the noise. In the multi-component version, polarization is used as an additional attribute for differentiation. Once single or multiple noise reference channels are established, coherent noise in the primary channel is canceled using data-adaptive least-squares multi-channel filter banks.
U.S. Pat. No. 4,556,962 attempts to attenuate the ground roll from a surface seismic source by placing a sensor close to the source to detect the interfering noise. The interfering noise is scaled, delayed and summed with signals from a more distant geophone array and then cross-correlated with the original vibrational source. This patent also suggests that an adaptive filter be used so as to modify the delayed signal to correspond more closely to that detected by the more distant geophone array. However, ground roll is in general of an inhomogeneous nature; due to dispersion and scattering from near surface anomalies the ground roll measured at one point increasingly deviates in character from that measured at another with increasing distance. Hence, the ground roll measured close to the source may be substantially different from that received by the geophone array, and the adaptive filter may not be able to deal with this. It is also difficult to measure seismic signals (ground roll) close to the source. Often the nearest offset is 100 meters. For close measurements, more robust sensors may be needed and detector xe2x80x98characterxe2x80x99 matching should be an important preliminary step.
In U.S. Pat. No. 4,890,264 a method is proposed for suppressing non-uniformly distributed noise generated by surface wave propagation, wind, and machinery. A number of horizontally sensitive geophones are distributed amongst the conventional vertically oriented geophones. The outputs of the surface wave detectors are utilized in conjunction with an adaptive filter to cancel the effects of the surface wave interference. This method for the suppression of ground roll is inherently a multi-component method, and cannot be used in conjunction with single component acquisition. In addition, it neglects the fact that some seismic body wave energy also is detected by horizontally sensitive geophones, and this may cause signal cancellation.
In UK Patent Application GB-A-2273358 linearly constrained adaptive beamforming and adaptive interference canceling beamforming is used for ground roll suppression. In linearly constrained adaptive beamforming, signals measured by an array of geophones are filtered and summed so as to preserve signals incident from a preferred direction while suppressing interferences incident from other directions. In applying adaptive interference canceling, the moveout differential between the seismic reflections and the ground roll is used to form primary and reference channels. The filtering is performed using a continuously adaptive method such as the LMS (least-mean-square) algorithm. The suggested application is in seismic while drilling, where the horizontal offset range is very small, so that the seismic reflections have an almost vertical angle of incidence and there is effectively a lot of data available from each source-receiver position since the roller cone drill bit used as the seismic source moves very slowly. The statistics of the noise then change very slowly, allowing stochastic gradient type of algorithms like the LMS to converge. However, in surface seismic experiments the ground roll present is often non-stationary and inhomogeneous. Therefore, stochastic gradient type of algorithms such as LMS may be too slow in converging.
U.S. Pat. No. 5,237,538 proposes a method for removing coherent noise from seismic data. In this method, first the moveout characteristics of the noise are identified. Next, a space-time gate containing the noise defined and extracted, and the moveout removed to flatten the noise train. Amplitudes and time variations are then removed from the gate. The coherent noise is estimated using conventional stacking. A single-channel Wiener filter is used to match the noise estimate to the noise in the data trace containing signal-plus-noise. Having subtracted the filtered noise estimate, inverse amplitude scalars are applied to undo the effect of amplitude equalization. The signal is then moveout restored into the original seismic record. This particular method for removing coherent noise from seismic data is an application of the well-known technique called Postbeamformer Interference Cancelling. It has some particular shortcomings for application for ground roll attenuation. First, the signal always leaks into the ground roll estimate, especially for shorter arrays. There is always a component of the signal present at the reference channel which is co-located in time with the signal in the primary channel. On the other hand, when the arrays are allowed to be longer, the dispersion present in the ground roll make it difficult to achieve effective beamsteering.
In marine seismic surveys, an acoustic source generates waves which travel through the water and into the earth. These are then reflected or refracted by the sub-surface geological formations, travel back through the water and are recorded by long hydrophone arrays which are towed near the surface of the water behind a seismic vessel. The hydrophones are mounted in streamer cables, or streamers. There are usually 1-12 streamers towed which are each several kilometers long. The streamers are made up of sections which may typically be 100-200 meters long; each section consists of hydrophones inside an outer skin which may be filled with oil, foam, or a more solid substance. Stress-wires and spacers form the internal skeleton of the streamer.
While the streamers are being towed behind the vessel, self-noise is generated due to a variety of sources. The lurching of the vessel, especially in rough seas, causes vibrations in the stress-wires which interact with the connectors and the oil-filled skin, generating bulge waves (or breathing waves) which propagate down the streamers. The pressure variations are detected by the hydrophones, adding and corrupting the detected seismic signals. As the streamer moves through the water, boundary layer turbulence causes pressure fluctuations at the outer skin wall, which are again coupled to the hydrophones.
Bulge waves may also be caused by eddy shedding under elliptical water motion about the streamer caused by wave action. A method of applying adaptive signal processing to the attenuation of bulge waves is described U.S. Pat. No. 4,821,241. There it is proposed to co-locate stress sensors with the hydrophones in the streamer. The stress sensors are responsive to mechanical stresses applied to the cable, but are substantially unresponsive to acoustic waves propagating in fluid media. The signal outputs from the stress sensors are combined with the signal outputs from the corresponding co-located hydrophones to cancel spurious signals due to bulge waves.
Another method of applying adaptive signal processing to the attenuation of bulge waves was described is described U.S. Pat. No. 5,251,183. In this patent it is proposed to use an accelerometer secured between the lead-in section of the streamer and the hydrophone. Intra-shot and inter-shot accelerometer and hydrophone signals are recorded. The method utilizes inter-shot and intra-shot adaptive processing loops. The inter-shot adaptive processing loop derives inter-shot complex weights from inter-shot accelerometer signals and inter-shot hydrophone signals. The intra-shot adaptive processing loop models bulge wave noise in the intra-shot hydrophone signals by combining the inter-shot complex weights with intra-shot accelerometer signals. Bulge wave noise attenuation is achieved by subtracting the intra-shot bulge wave noise model from the intra-shot seismic detector signals.
In accordance with the present invention, there is provided a method for filtering noise from discrete noisy seismic signals, comprising the steps of receiving signals using a plurality of receivers; determining propagation characteristics of the signals with respect to receiver locations; and filtering received signals using an at least partially adaptive filter such that signals having propagation characteristics other than the determined propagation characteristics are attenuated. The filtering step comprising the step of defining at least two independent sets of conditions (constraints) with a first set defining a desired (quiescent) response and a second set defining the propagation characteristics of signals to be preserved and the step of adapting filter coefficients of the filter subject to the independent sets of conditions (constraints) so as to minimize (optimize) the filter output for signals with propagation characteristics other than the determined propagation characteristics.
For the application of the invention, it is advantageous to define for the optimization process of the filter weights or coefficients a signal-dependent part (correlation matrix) and a signal-independent part. The signal-independent part usually comprises the constraints and is there often referred to as constraint matrix. Using this concept of a constraint matrix, an important aspect of the invention can be described as having within the constraint matrix a subspace which is defined by the desired quiescent response and one subspace which defines the regions of the protected signal. By making these two subspaces orthogonal, filter weights can be found which simultaneously impose both restrictions upon the filter response. As the constraint matrix effectively reduces the degrees of freedom of the filter available for the adaptation process, this aspect of the invention can be described as splitting the total number of degrees of freedom into a first part which is available for the adaptation process and a second part which is used to define the constraints. The degrees of freedom assigned to the constraints are split among those constraints which defines the desired response and a second set defining the temporal and/or spatial spectral content or the propagation characteristics of the signals to be preserved.
It is an advantage of the method to be not confined to narrow-band signals, but also applicable to wide-band seismic signals resulting in a filter that changes its response with the frequency of the input signal(s).
It is an important aspect of the invention that having derived a method of separating the desired quiescent response and the constraints relating to the region into orthogonal subspaces, any known method can be used for the adaptation process. Such known adaptive methods are known and described in the literature, e.g. LMS, RLS, LSL, FTF, etc.
According to a preferred embodiment of the invention, a filter bank comprising temporally and spatially local filters is used as the adaptive filter.
A filter bank can be defined as comprising M local multichannel adaptive filters with K channels, each of a length L. For most applications, the number L of coefficients is equal to or larger than three. The number of channels K and of individual filters M are preferably two or more.
The use of a filter bank for noise attenuation of seismic signals has been described in the International Patent Application WO97/25632. However, the present invention does not require defining a reference channel in order to calculate the adapted filter bank coefficients. No noise estimate enters the adaptation process. Therefore, the present method can be applied to noise contaminated seismic signals, where there is no independent measurement or estimate of the noise available.
According to one aspect of the invention, the coefficients of the filter are constrained such that its response corresponds to that of a beamformer with a specified look-direction.
According to another aspect of the invention the constraints are set such that the filter preserves signals from a range of look directions or of defined regions of the frequency-wavenumber domain. The region can be pre-selected depending on the nature, more specifically on the apparent velocity of the seismic signals. Certain limits of the velocity, such as 1500 m/s, define regions in the frequency-wavenumber domain.
A further aspect of the invention comprises the minimization of a cost functional using the approximation that the sum, weighted by window functions, of the output of adjacent filters of the M filters is equal when applied to the same signal in time regions where said window functions overlap. Preferably the method includes the step of multiplying M filtered estimates with temporal window functions. The application of the temporal window functions, and hence the resulting temporal windows, to the combined components ensures that the filtering process is local in time and allows the method adaptively to remove noise from the seismic data in accordance with a global optimization criterion. The data selection temporal window functions are preferably determined by two requirements, wherein the first requirement is that the sum over all windows at any given time equals unity and the second requirement is that only adjoining windows overlap. These requirements ensure that the global optimization of the filtered signal can be solved by use of an approximation in which for the sum over all time and all filters and all neighboring filters, the error function of a neighboring filter is replaced with the error function associated with the filter itself.
The application of the data selection temporal windows decouples the equation required to solve the optimization of the filtered signal.
According to yet another aspect of the invention, the response of the filter can be controlled by using a regularization parameter. The parameter as applied herein determines the relative weight of two components of the cost functional. One of the component of the cost functional can be defined as output power, while the other can be characterized as being essentially the white noise gain of the filter bank, i.e., the output of the filter in response to an input uncorrelated in time and space.
The noisy signal may be pre-processed before being passed to the adaptive filtering means by dividing the signal into frequency bands using a reconstructing filter, for example a quadrature mirror filter. This allows a reduction in the number of data points to be processed and also allows a reduction in the number of coefficients in the adaptive filtering means as effectively reducing the bandwidth of the original signal.
The invention is applicable for two-dimensional (2D) and three-dimensional (3D) seismic surveys, and can be used in land seismic, marine seismic including sea bottom seismic, and transitional zone seismic.
The method can be performed on stored data or on raw seismic data as it is acquired. Thus raw seismic data may be filtered according to the method at the data acquisition site. This ensures that a xe2x80x9ccleanedxe2x80x9d signal is available from the data acquisition site and may be downloaded directly from the site in this form. This reduces the amount of data sent for analysis off-site and reduces the costs and storage problems associated with accumulating sufficient quantities of noisy data for analysis off-site. The method can be advantageously applied to single-sensor recordings, i.e. to recordings prior to any group forming which combines the signals of two or more seismic sensors.
Although the description of the present invention is based on seismic signal processing, it can be applied to sonic signals as used for example for well logging applications. Specific seismic applications include swell noise or streamer noise attenuation, including streamer noise attenuation in a cross-flow acquisition, attenuation of ground roll or mud roll or other coherent noise from marine, land, or transition zone data, seismic interference canceling, i.e. filtering noise using the full aperture of a multi-streamer array, which is either towed in the water or deployed at the sea-bottom , or removal of sea-floor reflections from the notional source signature estimation, a technique described for example in the European Patent Application EP-A-066423. Other applications include noise suppression for various borehole seismic exploration methods, where either noise or signal has preferential directions. Known borehole seismic methods include seismic-while-drilling (SWD), vertical seismic profiling (VSP), look-ahead and look-around while drilling.